In plants such as an internal combustion engine and an exhaust purification system thereof, a plurality of physical quantities indicating the state of this plant is differentiated into physical quantities that are detected by way of a sensor (detected physical quantities) and physical quantities not detected by a sensor (non-detected physical quantities). Herein, physical quantities that cannot be directly detected by a sensor in principle, physical quantities that are not directly detected using a sensor for various reasons such as durability and cost, and the like are included in non-detected physical quantities. In a case of the necessity arising to control a plant based on such non-detected physical quantities, the estimated values thereof are calculated by a control device based on other detected physical quantities, basically.
As one concrete example of a non-detected physical quantity, a physical quantity related to an exhaust recirculation device such as the EGR amount and EGR rate in an exhaust purification system of an internal combustion engine can be exemplified. Technology related to the calculation of estimated values of the EGR amount and EGR rate, and control based on these estimated values is shown in Patent Document 1 and Patent Document 2.
Patent Document 1 illustrates a control device that calculates the estimated value of the EGR rate by way of a map in which the relationships between the aperture of various valves, intake new air amount, engine revolution speed, and EGR rate are established. With this control device, fluctuation in the amount of NOx discharged from the internal combustion engine is suppressed by regenerating a diesel particulate filter (hereinafter referred to as “DPF”) so that the estimated value of the EGR rate maintains a target EGR rate.
Patent Document 2 illustrates a control device that calculates an estimated value of the EGR amount based on the aperture of the EGR valve, the intake air amount detected by an air flow meter, and the like. With this control device, combustion noise of the engine in a transient operating state is reduced, and the emission amount of NOx is decreased by correcting the fuel injection timing and pilot injection amount based on the EGR amount thus estimated.
Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2008-106717
Patent Document 2: Japanese Unexamined Patent Application, Publication No. 2008-19782